Can someone definitively describe the difference between the sculpt flatten, polish, and smooth brushes?
Flatten tries to make all vertices align on one plane:


Smooth tries to align vertices to the average mesh surface:


Polish is basically a mixture of the two:


This is what I have observed from use and gathered from the Wiki. 

When pressing Shift while using the Brush tool, you switch to Smooth mode, and while pressing Ctrl you switch from Add to Subtract mode, and the opposite as well.